Butt fused polyethylene pipe is used extensively by domestic and foreign utility companies for distribution of natural gas. Although heat fusion of polyethylene is a reliable joining process, equipment malfunctions, environ mental conditions and deviations from the joining procedure can generate a variety of defects at the fusion interface. Strict quality control of the fusion joining procedure can significantly reduce the likelihood of fabricating a defective joint, but defective joints have been fabricated under ideal environmental conditions without any observable deviation from recommended joining procedures. To assure public safety and avoid the cost of excavating defective butt fusion joints that may develop a leak or catastrophically fail in service, an inspection tool for assessing the structural integrity of butt fused joints has been developed.
Inspection procedures currently practiced by the gas utilities include visual examination of the fusion bead and a variety of ultrasonic testing methods. These procedures involve a subjective interpretation of the results and have been shown to be unable to detect all of the types of critical defects that occur in butt fusion joints. Although an unacceptable visual appearance of the fusion bead is a consistent indicator of a defective fusion joint, a visually acceptable fusion bead does not reliably assure the absence of flaws at the fusion interface. It has been found that degradation of the mechanical integrity of butt fused joints often occurs before a defect is sufficiently large or severe to affect the appearance of the fusion bead. Thus, visual acceptability is a necessary but insufficient condition for assuring a good fusion joint. To supplement visual examination of the fusion bead, some utility companies ultrasonically inspect polyethylene pipe joints using manual contacting techniques. However, because of the nature of flaws in fused polyethylene, manual ultrasonic techniques do not reliably detect all the types of flaws in butt fused joints. The problems with these various manual ultrasonic inspection techniques were not fundamentally intrinsic to ultrasonics technology, but were found to be a matter of inadequacies specifically related to the particular manual techniques that were developed for this application.
For example, the manual ultrasonic inspection procedure of standard practice ASTM F600-78 involves manually moving a transducer around the pipe's circumference and maintaining proper alignment with respect to the weld interface while also attempting to interpret the observed signals on an oscilloscope. One difficulty with this procedure is that the constant attention of an experienced operator is required to simultaneously scan the joint, maintain acoustic coupling between the transducer and pipe, and interpret the signals on an oscilloscope. In addition, interpretation of the inspection results have been found to be very operator dependent, and the reliability of this method is directly dependent upon the skill of the individual operator. The acoustic coupling between the transducer and pipe is inefficient for several reasons which results in a poor signal to noise ratio with only a small portion of the acoustic energy being transferred into the pipe. This reduces the detectability of flaws and increases the chance of incorrect interpretation of the test results. Another problem with this approach is poor transducer design for this application. The curved surface of the pipe causes divergence of the ultrasonic beam, which, unless it is compensated for, results in decreased flaw sensitivity.
Ultrasonic flaw detection depends on the interaction of a propagating elastic stress wave with the discontinuities or gradients in material properties resulting from the presence of a flaw in the material. The wave interaction with a flaw is observed or detected indirectly by the manner in which the flaw perturbs one or more wave propagation parameters. The perturbation of the wave propagation parameters are typically observed as changes in wave speed, propagation direction due to specular or diffuse reflections, and attenuation due to scattering or absorption by the flaw. The material properties effected by the presence of a flaw are generally the density and the elastic moduli, but the specific properties effected depends on the physical nature of the flaw. For example, lack of bond produces a discontinuity in the dynamic tensile and shear moduli at the fusion interface since the unbonded interface cannot support tensile or shear stresses. The magnitude of the effect of these perturbations of the material properties on ultrasonic wave propagation is further influenced by the specific size, shape and orientation of the flaw.
Ultrasonic waves for inspecting pipe joints are usually generated by piezoelectric transducers. For high frequency ultrasonic waves to propagate from the transducer into the pipe wall, the transducer must be acoustically coupled to the pipe. Acoustic coupling of the transducer and pipe can be accomplished by using a variety of solids, fluids and viscous gels depending on the specific constraints of the application. The selection of the coupling medium directly influences the detectability of flaws because the properties of the coupling medium determine boundary conditions governing the wave propagation behavior at the pipe/couplant interface. The important properties of the coupling medium are its wave speed, density and attenuation.
Acoustic coupling between the ultrasonic transducer and the pipe surface is necessary because propagation of ultrasound in air is attenuated too severely for frequencies above 500 kHz and the mechanical impedance mismatch between air and a solid material causes nearly one hundred percent of the ultrasonic energy to be reflected from the surface of the solid material. Thus, almost no ultrasonic energy would be transmitted into the pipe with a concomitant loss of flaw sensitivity. Adequate acoustic coupling generally requires a fluid filled path between the transducer and the test object.
Conventional embodiments of acoustic coupling methods involving a liquid filled path are, in terms generally used by practitioners of ultrasonic testing, water jets, bubblers and immersion. The problems with the first two methods, water jets and bubblers, is that they often generate excessive noise in the ultrasonic signal which diminishes flaw detectability. A specific disadvantage of the water jet coupling method for a portable inspection system is that it requires a large volume of water and precise control of the water flow rate to maintain a hydrodynamically stable column of water.
Immersion which requires a reservoir of water large enough to contain the test object is generally incompatible with the constraints and requirements of field inspection. The system developed and applied in the present invention has combined the benefits and advantages of the small water volume of the bubbler and the absence of noise and efficient acoustic coupling characteristic of the immersion method. The new system involves attaching the scanner assembly to a container which opens in a "clam shell" fashion and when closed surrounds the scanner, transducer and fusion joint. The container is filled with a suitable fluid and emptied automatically during the inspection.
Non-destructive testing by ultrasonic wave propagation techniques in the past are revealed in the following U.S. Pats. Nos.: 4,472,975; 4,084,444; 3,958,451; 3,848,461. In these patents, various means are provided to propagate ultrasonic energy into pipe line materials and monitor signals. The reflected signals are sometimes termed echoes.
U.S. Pat. No. 4,084,444 - Lewis and U.S. Pat. No. 3,848,461 -Hetherington et al. shown apparatus and means for conveying an ultrasonic probe assembly in circular rotation along a steel tube or pipe. A transducer array is revealed to provide signal propagation at various angles to the tubing wall.
U.S. Pat. No. 4,472,975 - Beck et al. discloses an apparatus adapted to closely follow the contour of a pipe while carrying an ultrasonic transducer that is constructed to be angularly adjusted responsive to the selection of precise settings of the transducer holder, and the focal distance for the beam propagated by the transducer. A couplant cavity containing a couplant material is provided between the transducer and the test object.
U.S. Pat. No. 3,958,451 - Richardson discloses an ultrasonic test apparatus for detecting flaws in a weld on steel pipe including a carriage and electronic detection equipment with apparatus to present a graphic electronic representation of the signals returned. Signals returned are recorded by a light beam chart recorder. The recorder wave form is indicative of the characteristic internal structure of the pipe, but no means is provided to objectively evaluate the results nor to discriminate concerning an acceptable number of unobjectionable small "flaws" or irregularities.
The prior art addresses the problems associated with steel and magnetic pipe materials in which differences in the temperature, in the typical environmental temperature ranges at the inspection sites, have negligable effect on the ultrasonic wave propagation in the material.